In the processing of sulfur-containing exhaust gas intended for use in the recovery of sulfuric acid in contact process plants, it is conventional to extract sensible heat from such gas in a waste-heat boiler. That heat can be utilized as useful heat elsewhere, e.g. by producing steam for heating in the plant. Entrained dust must also be removed from the exhaust gas so that equipment downstream will not be clogged or rendered less effective by deposited dust.
This cleaning is usually effected at a temperature of 200.degree.-400.degree. C., generally in an electrostatic precipitator and/or a cyclone.
The exhaust gas is then passed through a scrubber for removal of constituents which are gaseous during the dry cleaning step but condense at lower temperatures. Such constituents are, e.g. H.sub.2 SO.sub.4 mist, arsenic oxide etc.
In the scrubber, the gas is usually sprayed with circulated scrubbing acid (H.sub.2 O+H.sub.2 SO.sub.4) and is thus adiabatically cooled to the dewpoint temperature of 60.degree. to 80.degree. C., depending on the H.sub.2 SO.sub.4 concentration of the scrubbing acid. The exhaust gas absorbs water as it is scrubbed and, upon leaving the scrubber, contains 200-400 g water per m.sup.3 (STP). As this water content is too high for the production of concentrated sulfuric acid, the exhaust gas must be cooled to a temperature of about 40.degree. C. and must be adjusted to a water content of about 60 g per m.sup.3 (STP).
Shell-and-tube heat exchangers, inter alia, are used for this cooling of the exhaust gas and must be made from carbon or lead in order to avoid corrosion. The shell-and-tube heat exchangers are usually installed in an upright position and the exhaust gas flows through the tubes from top to bottom. The cooling tubes may be provided with fins on the inside, in order to increase the heat transfer, and are contacted by flowing cooling water on the outside.
Because the temperature differences which are available are very small, such shell-and-tube heat exchangers must have a very large heat-excharge surface area and for this reason and owing to the special materials used to avoid corrosion are comparatively expensive. The higher the temperature of the cooling water which is available, the larger must be the heat-exchange surface area. Because the exhaust gas is at a given temperature, say 40.degree. C., very large and expensive gas coolers are required when the cooling water inlet temperature is as high as 25.degree. C.